Diminishing supplies of fossil fuels and growing environmental concerns continue to drive research for the development of alternative sources of energy. Each alternative energy source faces the same barriers of cost and efficiency. Fuel cells, based on the conversion of hydrogen fuel and oxygen from the air into electricity, offer unique potential as energy sources, especially for transportation applications. Vehicles powered by fuel cells would have essentially unlimited range because they could be refueled quickly and conveniently.
Only recently has there been significant attention directed to the potential of fuel cells for commercial vehicles. Their efficiency, power density and low emission potential have progressed over the past decade and they are beginning to show potential for zero-emission vehicles. The first fuel cell bus was completed in 1993 and several smaller fuel cell vehicles are now operating in Europe. Beyond transportation, fuel cells have been used to generate electricity in spacecraft, large military naval vessels and prototype power plants. Significant research is ongoing in Canada, Japan, the Netherlands as well as in the U.S. Fuel cells are expected to play an increasingly important role in decreasing fossil fuel dependency and improving air quality.
Fuel cells can operate on a variety of fuels including hydrogen and hydrocarbons such as methanol, ethanol and natural gas. Several fuels cell technologies are being considered, but currently the two most promising technologies for vehicular applications appear to be; 1) proton exchange membrane fuel cells, and; 2) phosphoric acid fuel cells.
In such fuel cells, the chemical energy from oxidation of a gaseous fuel is converted directly to electrical energy. Fuel cells differ from batteries in that reactants are supplied from an external source in fuel cells. The fuel and oxidizing gases are bubbled into separate chambers connected by a porous disk through which an electrolyte such as potassium hydroxide (KOH) can pass. Inert electrodes, often comprised of carbon, mixed with a catalyst such as platinum, are inserted into both chambers. When electrical connection is made between the electrodes and oxidation-reduction reaction takes place, forming water at the anode and liberating electrons upon the oxidation of hydrogen. These electrons migrate to the cathode, where they reduce oxygen.
Gas diffusion electrodes are currently the most important class of electrodes used in fuel cells of his type. The morphology and composition of the electrode material, the mass transport and electrical resistance characteristics of the material in the three phase region, and the distribution of catalysts and surfactants are all of critical importance in such fuel cells. The ability to tailor the electrical conductivity, cell size and connectivity and inertness to alter wetting offer attractive benefits for the use of tailorable carbon foam compositions and structures in such fuel cell applications.
A porous electrode that is inexpensive to produce and readily formed, conducts electricity well, promotes mass transfer of electrolyte and maintains consistent performance over its useful life is the holy grail of fuel cell research. The interest in reducing cost and weight and increasing the efficiency of the processes occurring in fuel cell operation is placing increasingly difficult demands on materials for electrode construction.
Fuel cell electrodes are commonly comprised of sintered metals, woven or non-woven carbon fiber mat or activated carbons. Each of these approaches has shortcomings. Firstly, electrodes base on sintered metals rely on porosity at particle interstices as mass transfer paths. Poor interstitial connectivity results in tortuous paths and reduced electrolyte transfer. Secondly, activated carbons and metals also suffer from changing performance with time. As these electrode materials absorb electrolyte or cell products, their efficiency changes. Carbon xerogels and aerogels are also being considered for electrode usage, but their durability in service and their cost pose significant hurdles. When confined to small spaces such as pores of membranes or porous electrodes or ion channels, electrochemical processes proceed quite differently than in the bulk state. One example of a phenomenon that can be detected in such environments is non-neutrality. Confinement reduces the number of ions in a micropore, and the counter ion concentration is not sufficient to balance the wall charges. Because of capillary and surface forces, flow and transport art quite different in this region than the bulk as well.
Accordingly, there still exists a significant requirement for improved fuel cell electrode materials that do not suffer from the previously described shortcomings.